1. Field of the Invention
This invention relates to the buck-boost converters, and more particularly, to buck-boost converters with soft switching in all of their semiconductor components.
2. Description of the Related Art
Bidirectional converters are increasingly being used in power systems with energy-storage capabilities (e.g., “smart-grid” and automotive applications), where they condition charging and discharging of energy-storage devices (e.g., batteries and super-capacitors). For example, in automotive applications, isolated bidirectional converters are used in electric vehicles (EVs) to provide bidirectional energy exchange between the high-voltage (HV) battery and the low-voltage (LV) battery, while non-isolated bidirectional converters are typically employed to optimize the traction inverter performance by pre-regulating its input voltage, as well as provide energy regeneration. Because a battery's operating voltage range depends on the battery's state of charge, achieving high efficiency across the entire operating voltage range of a battery is a major design challenge in bidirectional converter designs.
Non-isolated bidirectional converters are almost exclusively implemented by the buck-boost converter topology, such as that shown in FIG. 1. As shown in FIG. 1, voltage sources V1 and V2 each represent any component or a combination of components that can deliver or consume electric energy (e.g., capacitors, batteries, motors, motor generators, fuel cells, and passive loads). When the circuit in FIG. 1 operates as a boost converter, power is transferred from voltage source V1 to voltage source V2, where the voltage of voltage source V2 is greater than the voltage of voltage source V1. In a boost mode, control is achieved by (i) modulating switch S1 and (ii) operating switch S2 as rectifier D2. In a buck mode, power is transferred in the reverse direction, i.e., from voltage source V2 to voltage source V1. In a buck mode, control is achieved by (i) modulating switch S2 and (ii) operating switch S1 as rectifier D1. In this disclosure, to simplify the detailed description below, the label of a circuit element also represents its value. For example, the labels “V1” and “V2” of voltage sources V1 and V2 also represent their respective voltage values. Similarly, the label “L” of inductor L also represents its inductive value.
In a power converter, at higher power levels, the continuous-conduction-mode (CCM) operation is preferred over discontinuous-conduction-mode (DCM) operation because of CCM provides better performance. As described in U.S. Pat. No. 5,736,842, entitled “Techniques for Reducing Rectifier Reverse-Recovery-Related Losses in High-Voltage High Power Converters,” by M. M. Jovanovic (“the '842 patent”), the major limitations of CCM operations in high-voltage, high-power buck and boost converters at high frequencies are related to switching losses caused by reverse-recovery in the rectifiers and capacitive turn-on switching losses in the switches due to “hard” switching. Generally, in a unidirectional buck and boost converter, reverse-recovery-related losses can be virtually eliminated by using SiC or GaN rectifiers, instead of using the more cost-effective fast-recovery Si rectifiers. In the bidirectional buck-boost converter of FIG. 1, however, each of switches S1 and S2 may be implemented by a combination of a controllable switch and an antiparallel rectifier. For example, such a combination may include a SiC or GaN rectifier, and an IGBT. Alternatively, such a combination may include an emerging SiC and GaN rectifier and a MOSFET switch. The IGBT implementation is limited to a relatively low frequency, due to the relatively limited switching speed of an IGBT, which increases the size of the converter. However, at this time, the SiC or GaN switch implementation is not attractive, primarily due to increased cost, as well as a lack of sufficient in-the-field reliability data. Today's most cost-effective, high-frequency implementations can employ high-voltage Si MOSFETs only when the reverse-recovery-related losses of the slow parasitic antiparallel body diodes are significantly reduced.
A technique that has been shown to virtually eliminate reverse-recovery losses in unidirectional non-isolated converters is described in U.S. Pat. No. 6,987,675, entitled “Soft-Switched Power Converters,” by M. M. Jovanovic et all. (“the '675 patent”). The technique described in the '675 patent employs an active snubber that controls the current turn-off rate of a rectifier. The active snubber achieves a reduction in the reverse-recovery losses, as well as creates conditions for zero-voltage switching in a power-regulating switch. In addition, this technique also achieves soft-switching of the snubber switch by turning it off at zero current.